The present invention relates to sliding parts, for example, for use as threaded parts such as a nut and a screw for providing a screw-fastening type pipe joint which are adapted for screw-thread engagement with each other, a sliding bearing and a shaft thereby supported, two platelike members slidable on each other, etc.
For example, the nut and screw of a screw-fastening type pipe joint are each made of stainless steel for use in the piping for semiconductor production equipment, piping for atomic power plants and piping for chemical plants. When the nut is screwed on the screw to provide the pipe joint, a great load acts on the threaded portions of the two members, so that the joint has the problem that seizure occurs between the threaded portions since no lubricant is usable.
Accordingly, an Ag plating layer is conventionally formed on the surface of threaded portion of the nut to preclude such seizure. Since Ag forms no alloy with Fe, Ag is unlikely to cause seizure when the nut is rotated on the screw, further exhibiting the advantage that the fastening torque is low and stable owing to reduced frictional resistance.
However, Ag is a metal liable to gall. The Ag plating layer wears away to produce a large amount of particles owing to the sliding contact between the threaded portions when the nut is driven on the screw to provide a pipe joint of the screw-fastening type. Further when the connected pipes are separated by removing the pipe joint and then connected together again by the joint, the particles produced are likely to ingress into the piping. When the particles ingressing into the piping are not smaller than 50 .mu.m in size and adhere to the seat of a valve provided at an intermediate portion of the piping, a leak is likely to occur while the valve is closed even if the seat is made of a resin having a relatively high sealing property. If the seat is made of metal, a leak will occur from the closed valve even when the particles are about 10 .mu.m in size. Especially the piping for semiconductor production equipment encounters the problem that the particles ingressing into the piping, even if about 0.1 .mu.m in particle size, adhere to wafers, causing short-circuiting or resulting in altered characteristics.